1. Field of the Invention
The present invention relates generally to an automatic transmission, and more particularly, to a slip control device of a fluid coupling provided in an automatic transmission.
2. Description of the Related Art
A torque converter (fluid coupling) of an automatic transmission as disclosed in Japanese Patent Publication (examined) No. 61-50179 is provided with a lockup clutch for directly coupling the input and output sides thereof to each other and a slip control device. The slip control device controls the locking force of the lockup clutch in a specified speed range in order to improve power transmission efficiency, while a torque increasing function and a torque fluctuation absorbing function are appropriately kept by the torque converter. Thus, the lockup clutch is allowed to be in a slip condition intermediate between a complete released condition and a complete locked condition. More specifically, the slip control device includes a slip control means for comparing a target amount of slip with the actual amount of slip corresponding to the difference between the number of rotations of the input side of the torque converter (engine speed) and the number of rotations of the output side thereof (turbine speed), thus performing feedback control of the locking force of the lockup clutch so that the actual amount of slip may approach the target amount of slip.
Furthermore, according to this disclosure, the turbine speed of the torque converter is indirectly detected on the basis of the speed of an output shaft of a transmission gear mechanism and a series of mating gears. When the turbine speed detected indirectly is used for a feedback control, the feedback control is stopped during the speed change to prevent an erroneous control caused by the difference between the indirectly detected turbine speed and the actual turbine speed. Accordingly, the locking force of the lockup clutch set immediately before a speed change order has been issued is maintained until the speed change is terminated.
However, during the speed changing operation, even though the locking force of the lockup clutch is a value detected immediately before the speed change is made, a shock due to the speed change cannot be sufficiently softened or the acceleration feeling is lowered because the locking force of the lockup clutch is too strong. More specifically, during a shift-up operation in which the locking condition of the friction elements of the transmission mechanism is switched according to a shifting order, the turbine speed is reduced. If the locking force of the lockup clutch is substantially the same as that detected before shifting is made, the engine speed is comparatively rapidly reduced with the reduction of the turbine speed, thereby producing a shock and lowering the feeling of acceleration.